1. Field of the Invention
The present invention relates to a PTC (positive temperature coefficient) composition produced by a grafting method and used as a self-resetting overcurrent protection element in electrical circuits.
2. Description of the Prior Art
PTC composition are used for self-resetting overcurrent protection elements. In the event of excessive current or voltage in a circuit an increase in the resistance of the PTC overcurrent protection element limits the current which is permitted to pass therethrough.
Such overcurrent protection elements suffer the drawback that, after being subjected to an overcurrent condition, their resistance does not return to the value it had before the overcurrent condition.
A further problem is that it is difficult to disperse conductive particles, especially carbon black, into a crystalline polymer binder. The carbon black does not enter the polymer structure as separate particles but as aggregates containing numerous particles that become distributed unevenly throughout the element. As a result, the internal resistance of the PTC composition element is non-uniform.
Consequently, power dissipation is unevenly distributed through the material, thus causing localized heating, and threatening premature failure of the element.
Japanese Patent Laid-Open No. 110702/1989 discloses an attempt to overcome problems caused by uneven distribution of particles in the element. The prior art teaches formulating a PTC composition self-resetting overcurrent protection element using a grafting method. In this method, an organic peroxide, serving as a grafting agent, is added to colloidal graphite and carbon black. The mixture is blended into high-density polyethylene. This converts a part of the high-density polyethylene having an unpaired electron to the polyethylene radical due to thermal decomposition of the organic peroxide. The aggregates of carbon black are disintegrated and dispersed into the polyethylene by grafting the polyethylene having an unpaired electron to the colloidal graphite and carbon black. The mixture is then shaped and cross-linked to stabilize the composition and give it immunity to changes in resistance value caused by repeated application of voltage.
This grafting method consists of the following sequences:
transformation of organic peroxide to the RO.cndot. radical by thermal decomposition;
generation of the polyethylene having an unpaired electron P.cndot. by removal of the tertiary hydrogen atoms located at the respective branching points of side chains of the polyethylene by RO.cndot. radical; and
coupling the polyethylene having an unpaired electron P.cndot. with the phenoxy radical on the surfaces of the particles of colloidal graphite and carbon black.
This process results in the grafting of the polyethylene to the colloidal graphite and the carbon black.
Consequently, in order to graft polyethylene easily, it is preferable to have a large number of tertiary hydrogen atoms on the backbone of the polyethylene having an unpaired electron for generating polyethylene radical P.cndot.. For use in PTC composition overcurrent protection elements, a resistance/temperature characteristic whose resistance value changes radically when the temperature reaches a specified point is more desirable than one whose resistance value changes gradually with a rise in temperature. Polymers having the former characteristics are highly crystallized polymers, such as high density polyethylene. However, highly crystallized polymers lack the desired large quantity of tertiary hydrogen atoms in their main chains. For example, high density polyethylene has approximately one tertiary hydrogen atom for every 1,000 carbon atoms on the backbone of the polymer. Therefore, when a highly crystallized polymer such as, for example, a high density polyethylene is used, the ability to graft to carbon black is poor due to the small number of available polymer having unpaired electrons P.cndot.. As a result, it is difficult to disperse carbon black uniformly into the polymer. As a result of poor dispersion of the carbon black, the resistance of a PTC composition overcurrent protection elements is uneven. Localized heating during current limiting degrades the ability of the element to return to its original resistance value after repeated current limiting actions.